If ya headunit has a sub pre-out run the 6x9s directly from the head unit if your only running 1 amp, you will have more control over sound quality. As for which sub to use,well that depends on which style of music you normaly listen to,if ya into dub,reggae n dancehall stylee or anything with deep heavy bass go for the 15", if it's dubstep, drum n bass,electro or house go for the 12". Try and keep ya tops n mids seperate from ya subs.